Fluid-attenuated inversion recovery (FLAIR) is one of inversion recovery techniques. Particularly, FLAIR can be used in brain imaging to suppress cerebrospinal fluid effects on the image.
FIG. 1 is a diagram illustrating a T2-weighted image and a FLAIR image generated by the FLAIR technique. Both the T2-weighted image in the left side of the figure and the FLAIR image in the right side of the figure are generated by using long repetition time (TR) and long echo time (TE). Accordingly, except that a cerebrospinal fluid signal is suppressed and appears black on the FLAIR image, the white/gray contrasts of the two images are similar. By using the FLAIR image on which the cerebrospinal fluid signal is suppressed, it is possible to observe lesions accurately and diagnose a disease that emerges near a ventricle.
FIG. 2 is a diagram for describing a pulse sequence and T1 relaxation after the application of an inversion pulse of 180 degrees according to an inversion recovery technique applied to a magnetic resonance imaging device.
A pulse sequence of a spin echo image starts from an excitation pulse. In case of a pulse sequence according to the inversion recovery technique, however, an inversion pulse of 180 degrees is applied prior to an excitation pulse. Immediately after the inversion pulse of 180 degrees is applied, net magnetization of a biological tissue is completely inverted to a minus (−) direction of the longitudinal axis. Afterwards, T1 relaxation occurs according to characteristics of each tissue and magnetization in a plus (+) direction of the longitudinal axis starts to appear.
During this process, there is a time point when the net magnetization of the tissue in the direction of the longitudinal axis becomes zero (0). A time period from the time point when the inversion pulse of 180 degrees is applied to the time point when the net magnetization becomes zero (0) is referred to as inversion time (TI). By way of example, fat has inversion time of 150 ms; white matter, 300 ms to 400 ms; gray matter, 600 ms to 700 ms; and cerebrospinal fluid, 2000 ms to 2500 ms.
Thus, after the inversion pulse of 180 degrees is applied, an excitation pulse is applied after the lapse of time as much as the inversion time of a tissue intended to be suppressed. That is, if an excitation pulse is applied after the lapse of 2000 ms to 2500 ms after the inversion pulse of 180 degrees is applied, a FLAIR image on which cerebrospinal fluid signals are suppressed can be generated.
In the conventional FLAIR technique as mentioned above, flow signals are selectively eliminated by using the inversion time after the application of the inversion pulse. As the inversion time increases, however, the time required to acquire data also increases, raising a problem in using the technique clinically.
In this regard, U.S. Pat. No. 5,528,144 (entitled “Interleave slab inversion for weighted throughput in fluid attenuated inversion recovery imaging) discloses a technique of improving a processing speed by reducing data acquisition time through the use of a multi-slice technique when generating an image on which a cerebrospinal fluid signal is suppressed.